Eucalyptus camaldulensis, the river red gum, is a tree of the genus Eucalyptus. It is one of around 800 in the genus. It is a plantation species in many parts of the world, but is native to Australia, where it has the most widespread natural distribution of Eucalyptus in Australia,[1] especially beside inland water courses. Oddly, it is named for a private estate garden near the Camaldoli monastery near Naples (L'Hortus Camaldulensis di Napoli), from where the first specimen came to be described. Material from this tree was used by Frederick Dehnhardt, Chief Gardener at the Botanic Gardens in Naples, to describe this species in 1832.[2]

It is a familiar and iconic tree seen along many watercourses right across inland Australia. The tree produces welcome shade in the extreme temperatures of central Australia, and plays an important role in stabilising river banks.

The tree can grow to 45 metres (148 ft) tall; it has smooth bark, ranging in colour from white and grey to red-brown, which is shed in long ribbons.[3] The tree has a large, dense crown of leaves. The base of the bole can be covered with rough, reddish-brown bark.[4] The juvenile and adult leaves are stalked, with the adult leaves broad at the base, tapering to the tip. The adult leaf colour is a dull blue-green. The leaf also contains several to many oil-producing glands in the un-veined areas of the leaf.[5] It is fast growing, and usually grows to 40 to 45 metres (131 to 148 ft) in height, depending on its location.[6][7] The tree grows straight under favourable conditions, but can develop twisted branches in drier conditions.[4]

River reds and many other eucalypts have an ominous nickname, "widow maker", as they have a habit of dropping large boughs (often half the diameter of the trunk) without warning.[8] This form of self-pruning may be a means of saving water or simply a result of their brittle wood. This is also an efficient way of attracting wildlife that live in the holes formed, which gives the red gum a source of natural fertiliser[citation needed].

Eucalyptus camaldulensis consists of two variations and one subspecies: E. camaldulensis var. camaldulensis, and E. camldulensis var. obtusa. The subspecies Eucalyptus simulata, found in North Queensland, has been recognised as a hybrid of var. obtusa and Eucalyptus tereticornis.[9]

The dry river beds of central Australia have sufficient underground water flow to sustain the trees.

E. camaldulensis is commonly found along many waterways within Australia, being the widest natural distribution of any eucalyptus species. The species is inextricably linked to watercourses. In fact, in only a few locations in Australia will the species occur away from water courses.[7]Eucalyptus camaldulensis var. camaldulensis is found in the Murray-Darling catchment area, whereas Eucalyptus camaldulensis var. obtusa is found outside the Murray-Darling catchment.

The species can be found along the banks of watercourses, as well as the floodplains of those watercourses. Due to the proximity to these watercourses, river red gum is subject to regular flooding in its natural habitat. River red gum prefers soils with clay content. The trees not only rely on rainfall but also on regular flooding, since flooding recharges the sub-soil with water.[10]

The association of the river red gum with water makes the tree a natural habitat choice, indeed sometimes the only choice in drier areas, for other species. The trees provide a breeding habitat for fish during the flooding season, which also benefits aquatic bird life that depend on fish as a food source during their own breeding season. Wilson,[7] who examined the management of river red gums in NSW, suggests shelter is provided for fish in rivers and streams by fallen branches from the river red gum. The "snags" formed when river red gums fall into rivers such as the Glenelg, are an important part of river ecosystems, and vital habitat and breeding sites for native fish like river blackfish. Unfortunately most snags have been removed from these rivers, beginning in the 1850s, due to river-improvement strategies designed to prevent hazards to navigation, reduce damage to in-stream structures, rejuvenate or scour channels, and increase hydraulic capacity to reduce flooding.[11] However, the Murray–Darling Basin Commission has recognised the importance of snags as aquatic habitat, and a moratorium on their removal from the Murray River has been recommended.[12]

Hollows start to form at around 120–180 years of age, creating habitat for many wildlife species, including a range of breeding and roosting animals such as bats, carpet pythons, and birds.[7] The dense foliage of the tree also provides shade and shelter from the sun in drier areas.

River red gums contribute to the provision of nutrients and energy for other species through leaf and insect fall. This is especially important to the ecology in areas of low nutrients.[7] The tree's preferred habitat of floodplains and watercourses also gives it the role of flood mitigator, which slows silt runoff.

The flower begins as an "invaginated receptacle".[5] The operculum, or cap, protects the interior of the flower bud, as the male and female parts develop.

The male parts of the flower consist of the stamen, a slender filament, and the anther, two pollen sacs located at the top of the stamen. The anther sacs open into longitudinal slits to release their pollen. These filaments will extend to encircle the receptacle during flowering.

The female parts of the flower, the ovaries, are contained in ovary chambers. These chambers are separated from the receptacle containing the male parts by a disc. From the top of the ovaries a structure called the style extends into the receptacle, to form the stigma.

During flowering, pollen from the anthers can fall onto the stigma. This can occasionally lead to self-pollination, although the stigma does not become receptive until a few days after the operculum has been detached by the expanding stamens, and the flower's pollen has already been released.[6] Fertilisation will therefore occur with other flowers on the same tree or other flowers on a different tree. Insects, birds, and small mammals help in the pollination of other flowers.[5][6]

After flowering, the stamens will detach. The fruit is the part of the flower that remains after fertilisation, which enlarges, dries, and becomes woody. Triangular valves in the fruit will open, dispersing yellow, cuboid seeds. When seeds are shed from a tree, most fall onto the ground below the crown, with some seed carried by the wind and water. Dissemination occurs mostly in spring and summer,[10][14] while natural flooding occurs during winter and spring.[10] As the tree is inextricably linked with waterways, seed dispersion would logically be facilitated by floodwater. There is some contention in this theory, however, where the CSIRO describes an experiment that demonstrated seeds were found to sink after only 36 hours.[14] It would also seem that as the seeding and flooding do not entirely coincide, it could be inferred that the conditions for germination, such as damp soil and plenty of sunlight, are more important in the continuation of the species than seed dispersal by means of floodwater. Seeding during the flooding season would prevent desiccation of the seed, which is the main cause of a seed's failure to reproduce.[6] Despite this apparent evolutionary advantage of the species living near watercourses to avoid seed desiccation, many seeds will be produced within a Camaldulensis forest before one will grow to its own reproducing stage. A gap in the forest must be available for the germinated seed to receive adequate sunlight.[6]

The formation of the famous Barmah red gum forests is due to a relatively recent geological event in the Murray-Darling Basin involving the Cadell Fault.

River red gum seeds germinate readily after floods and require regular spring floods throughout their life to survive. In the Murray-Darling Basin, such floods are now rare due to river regulation for irrigation, and as a result, 75% of River red gums in the lower Murray are stressed, dead or dying.

The largest remaining stand of river red gum is the 65,000 ha (160,000 acres) Barmah-Millewa forest straddling the border of Victoria and New South Wales, due north of Melbourne. It retains enormous cultural significance to the Indigenous traditional owners, the Yorta Yorta Nation. Like many stands of river red gum, the Barmah-Millewa has been drastically altered by over 100 years of timber harvesting. There is a paucity of old hollow-bearing trees which provide habitat for rare and threatened fauna such as the superb parrot, brush-tailed phascogale and inland carpet python. (Though these species are currently not under threat.) The increasing scale of logging machinery is creating large areas of intensive soil disturbance and bare earth, which is likely to increase weed invasion and increase the likelihood of the extinction of rare understorey plants.

About 25,000 years ago, displacement occurred along the Cadell fault, raising the eastern edge of the fault (which runs north-south) 8–12 metres (26–39 ft) above the floodplain. This created a complex series of events. A section of the original Murray River channel immediately behind the fault was abandoned, and exists today as an empty channel known as Green Gully. The Goulburn River was dammed by the southern end of the fault to create a natural lake. The Murray River flowed to the north around the Cadell Fault, creating the channel of the Edward River which exists today and through which much of the Murray River's waters still flow. Then the natural dam on the Goulburn River failed, the lake drained, and the Murray River avulsed to the south and started to flow through the smaller Goulburn River channel, creating "The Barmah Choke" and "The Narrows" (where the river channel is unusually narrow), before entering into the proper Murray River channel again.

The primary result of the Cadell Fault however is that the west-flowing water of the Murray River strikes the north-south running fault and diverts both north and south around the fault in the two main channels (Edwards and ancestral Goulburn) as well as a fan of small streams, and regularly floods a large amount of low-lying country in the area. These conditions are perfect for river red gums, which rapidly formed forests in the area. Thus the displacement of the Cadell Fault 25,000 years BP led directly to the formation of the Barmah river red gum forests.

E. camaldulensis readily germinates from both fresh seed and seed stored in cool dry conditions. It quickly toughens up and can withstand drought even whilst in forestry tubes. It makes an excellent bonsai and will readily regrow both from the base and from epicormic buds.

Red gum is so named for its brilliant red wood, which can range from a light pink through to almost black, depending on the age and weathering. It is somewhat brittle and is often cross-grained, making hand working difficult. Traditionally used in rot resistant applications like stumps, fence posts and sleepers, more recently it has been recognised in craft furniture for its spectacular deep red colour and typical fiddleback figure. It needs careful selection, as it tends to be quite reactive to changes in humidity (moves about a lot in service). It is quite hard, dense (about 900 kg/m3 (1,500 lb/cu yd)), can take a fine polish and carves well. It is a popular timber for wood turners, particularly if old and well-seasoned.

It is also popular for use as firewood. Significant amounts of Victoria and NSW's firewood comes from red gums in the Barmah forest.

The wood makes fine charcoal, and is successfully used in Brazil for iron and steel production. In addition, this plant is used for beekeeping in Brazil and Australia. Recently, it has been used to produce decks (Patagonian cherry) and wooden floors (Andean cherry).

The speed of growth of the tree makes it a useful plantation timber. Apiarists also use the tree’s flowers for honey production. E. camaldulensis is important in supporting the ecology of its habitat through providing food, and shelter for breeding. Culturally, the species is an iconic part of Australia. Its leaves have appeared on Australian stamps[15] and is widely recognised due to its widespread range. The use of the waterways for recreation also occurs within the habitat of the river red gum, again due their fundamental link to watercourses and floodplains. Recreation would vary with the season.[7]

The predilection of the river red gum for waterways has been a successful evolutionary niche. This has resulted in a large population and range for the species, and so it is not considered endangered. Changes in its habitat, however, could be detrimental not just for the tree, but also for species that depend on the tree for their own survival. These changes include grazing, and water regulation for irrigation purposes. For example, grazing reduces the ability of the species to regenerate, as stock eat or trample the seedlings. However, grazing may aid regeneration by removing thick ground cover.[7]

In regards to water regulation, there are two problems. One is the timing of the water flow, and the other is the minimisation of natural flooding.

Regulation causes flooding to be decreased during the winter and spring months, and water more consistently flows during the summer and autumn months. Since the river red gum disperses its seed during spring, regulating the water may affect the species' ability to disperse using water as a dispersing agent, especially in floodplain red gum forests. Natural water run-off can also be affected, leaving some trees permanently flooded due to the build-up of water behind dams, or the permanent water flow. Neither can seeds germinate in constantly flooded areas.

Infrequent flooding due to water regulation provides inadequate water to recharge the floodplain subsoils that river red gums depend on. This will result in stunted tree growth, death of existing trees, and poor conditions for seed germination. Lack of flooding in floodplain areas will change the suitability of river red gum habitat as a breeding ground and food source for other species. Indeed, extinctions of some species have already occurred in river red gum habitats in the Murray-Darling catchment.[7]

It has been recognised since around the early 1980s that managing water more effectively would ensure the maintenance of river red gum habitat. Water management would include the removal of subsidies for irrigation, issuing water licenses, and the flooding of forests in suitable seasons.[10] The Australian Federal government is currently pursuing this course of action.